In general, a cylindrical roller bearing is configured in a manner that, in order to guide cylindrical rollers in the circumferential direction of a raceway ring, a flange portion is provided at the end portion of the raceway ring, and the end face of each of the rollers is made in slide contact with a roller guide-surface formed at the inner surface of the flange portion. A cylindrical roller bearing used for the transmission etc. of an automobile may be used in a state of being applied with an axial load in addition to a radial load. When the cylindrical roller bearing rotates while being applied with the axial load, a local stress such as an edge load acts on a portion where the end face of the cylindrical roller contacts with the roller guide-surface of the flange portion thereby to generate a large friction at the contact surface. When the friction becomes remarkable due to the high speed rotation, wear and seizure are caused and so there may arise a case that the bearing is broken in an early stage.
Thus, in order to make a contact surface pressure between the end face of the roller and the roller guide-surface of the flange portion as uniform as possible, the end face of the roller or the roller guide-surface of the flange portion is slightly tapered or crowned so as not to cause a large edge load.
As a technique of preventing or reducing the seizure of a cylindrical roller bearing, there is a technique of improving the surface roughness of the contact surfaces of the roller and the flange portion. There is another technique in which a crowing is provided at the end face of the roller so as to contact with the end face of the roller at the center position of the height of the flange thereby to prevent the contact point of an elliptical shape from falling into a relief groove or protruding largely to the outside of the flange height, for example.
As a conventional technique, there is known a roller bearing in which the surface roughness of the end face of a roller and the flange portion to be contacted to each other is improved thereby to prevent the lubrication failure (see Japanese Patent Unexamined Publication No. JP-A-7-42746, for example). Further, there is known another roller bearing in which a working grain is provided in a direction almost perpendicular to the relative movement direction in order to improve the lubrication efficiency (see Japanese Patent Unexamined Publication No. JP-A-7-91452, for example). Furthermore, there is known still another roller bearing in which minute recess portions are provided at the contact region of a flange portion contacting with the end face of a roller thereby to intend to maintain the forming of an oil film (see Japanese Patent Unexamined Publication No. JP-A-6-241235, for example). Furthermore, there is known still another roller bearing in which, in order to reduce an amount of heat generated by the sliding operation, the shape of a roller is changed so that the position where the roller contacts with a flange portion is made same as the outer diameter surface of the roller (see Japanese Patent Unexamined Publication No. JP-A-9-236131, for example).
Furthermore, there is proposed a technique in which oil films are surely formed within a roller and a flange surface at the rolling contact portion thereby to prevent or reduce seizure by placing the intersection position of the chamfering portion and the plane portion of the roller end face on the radial-direction small diameter side in an outer ring flange portion and the radial-direction large diameter side in an inner ring flange portion, rather than the intersection point of the flange surface and a relief portion for grinding (see Japanese Patent Unexamined Publication No. JP-A-2002-181053, for example).
However, the improvement of the surface roughness and the provision of the working grain as respectively disclosed in the JP-A-7-42746 and JP-A-7-91452 cause a problem of the cost-up and a problem in the stability of quality due to the particular processing equipment and the extension of the processing time. On the other hand, even in the case where the roller and the flange portion are changed in the particular shapes as disclosed in JP-A-6-241235 and JP-A-9-236131, respectively, these techniques are not effective since the processing of the roller and the additional processing of the inner and outer rings raise the costs thereof.
The cylindrical roller bearing disclosed in JP-A-2002-181053 is configured in a manner that the flange surface is crowned or that the contact portion between the inner ring flange portion and the roller end face becomes a contact portion between the outer diameter surface (inner ring flange surface) of a cone having a point as an apex and the spherical surface (the end face of the roller). However, the direction of the major axis of the contact ellipse caused at the contact portion between the roller and the flange portion is the radial direction of the inner ring of the bearing. Thus, there arises a problem that the contact ellipse may protrude from the relief groove of the inner ring flange surfaced or the outer diameter of the flange surface depending on an amount of the axial load, so that an edge load is generated at the boundary surface between the contact ellipse and the relief groove or the boundary surface between the contact ellipse and the outer diameter portion of the flange portion.
Further, in order to prevent the contact ellipse from protruding from the relief groove of the flange surface, it is considered to raise the center position of the contact ellipse which is the contact surface between the roller and the flange portion on the outer diameter side of the flange surface. However, in this case, since the slippery degree between the roller and the flange portion becomes large, a heat value increases, whereby there arises such problems that the temperature of the bearing increases, scoring or seizure etc. of the flange portion occurs.
In order to solve the aforesaid problems, an object of the invention is to provide a cylindrical roller bearing which can improve a seizure property and improve an allowable rotation speed without increasing the size of a bearing.